1. Field of the Invention
The present invention relates to a method for manufacturing a single crystal silicon solar cell, and a single crystal silicon solar cell.
2. Description of the Related Art
Solar cells produced using silicon as a principal raw material are classified into single crystal silicon solar cells, polycrystal silicon solar cells, and amorphous silicon solar cells, depending on their crystallinity. Among these types, single crystal silicon solar cells are formed by slicing a single crystal ingot obtained by crystal pulling into wafers using a wire saw, processing each wafer to a thickness of 100 to 200 μm, and forming pn junctions, electrodes, a protective film, and the like on the wafer.
Polycrystal silicon solar cells are formed by producing a polycrystal ingot, not by crystal pulling, but by crystallization of molten metal silicon in a die, slicing the polycrystal ingot into wafers using a wire saw as with single crystal silicon solar cells, processing each wafer to a thickness of 100 to 200 μm, and forming pn junctions, electrodes, a protective film, and the like on the wafer as with single crystal silicon solar cells.
Amorphous silicon solar cells are formed by decomposing a silane gas in a gaseous phase through discharge using, for example, a plasma CVD method, to form an amorphous silane film on a substrate, simultaneously performing the steps of forming pn junctions and forming a film by adding diboran, phosphine, or the like to the amorphous silane film as a dopant gas and simultaneously depositing a film, and forming electrodes and a protective film on the substrate. The amorphous silicon solar cells are advantageous in that a sufficient thickness of the amorphous silicon layer is around 1 μm, which is approximately one hundredth that required in crystal solar cells. This is because amorphous silicon absorbs incident light as a direct-transition type material, and therefore, exhibits a light absorption coefficient higher than those of single crystal silicon and polycrystal silicon by approximately one order-of-magnitude (Kiyoshi TAKAHASHI, Yoshihiro HAMAKAWA, and Akio USHIROKAWA, “Taiyo-ko Hatsuden (Photovoltaic Power Generation)”, Morikita Shuppan, 1980, p. 233). Given that the amount of power produced by solar batteries worldwide has recently exceeded one gigawatt on an annual basis, and that amount will further increase, there is a high expectation for thin-film amorphous silicon solar cells that allow effective use of resources.
However, the fabrication of amorphous silicon solar cells includes the use of a high-purity gas such as silane, disilane, or the like as a raw material, and part of the gas is deposited on regions other than the substrate inside a plasma CVD apparatus. In view of these circumstances, the effective utilization rate of resources cannot be determined by simple comparison between the required silicon-layer thicknesses in amorphous silicon solar cells and crystal solar cells. Moreover, the conversion efficiency of amorphous silicon solar cells is around 10%, whereas that of crystal solar cells is around 15%. In addition, amorphous silicon solar cells still suffer from deterioration of the output characteristics under light irradiation.
For these reasons, various attempts have been made to develop thin-film solar cells using crystal silicon materials (Kiyoshi TAKAHASHI, Yoshihiro HAMAKAWA, and Akio USHIROKAWA, “Taiyo-ko Hatsuden (Photovoltaic Power Generation)”, Morikita Shuppan, 1980, p. 217). One such example is to deposite a polycrystal thin film on an alumina substrate, a graphite substrate, or the like, using trichlorosilane gas, tetrachlorosilane gas, or the like. Since such a deposited film has many crystal defects and lowers the conversion efficiency as it is, it is necessary to improve the crystallinity by zone melting in order to enhance the conversion efficiency (see, for example, Japanese Unexamined Patent Application Publication No. 2004-342909). However, even with such a method employing zone melting, there have been problems such as leakage current at grain boundaries and deterioration of photocurrent response characteristics at long wavelengths due to a shortened lifetime.